If you have any woodworking skills and the time to do it I would strongly consider going DIY for the subs. Then you can design them the size and shape you want and finish with the material of your choice for less than it costs to buy premade. My room is almost the same dimensions as yours, 11.5ft x 21ft x 8ft, and two Dayton Ultimax 15" subs in the same ported box under my screen works beautifully for me. Total spent for subs, amp, and building materials was only about $800.

I think I will be going with the SVS Plus. They are now $300 cheaper shipping included. Last year they were almost$400 more expensive.

I perfer an unsealed sub for movies anyways. Sealed I like for music, however, if I decide to upgrade someday to the M100 I won't need a sub for music and the Anthem has that option for music playing to cut out the LFE.

My vote, provided you have the skills and tools, would be the DIY'er route, if the polls are still open. There's both ready-to-assemble kits or fully diy build-ups with lots of info floating around online for assistance.

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The ksw-15 rumbled a lot and shook the hole house, dishes were shaking, you could feel the bass was strong but lacked the musical punch or snap, it felt muddy and seem to lack a lot of info on the bass spectrum compared to the ep500. With the ksw-15 with a movie you could feel the explosions but with concerts it wasn't all that pleasurable.

The ep500 v4 didn't shake the house, had the volume on the sub 70% up, it seemed anemic compared to the ksw-15, it felt very precise in a concert dvd's but a tad weak with action movies I expected a little more from this sub, best description it's missing that over powering bass effect. I'm debating if I should return it, perhaps give it a few more days and play with settings, debating if I should get the svs 13 ultra or a second ep500

After playing around with rear settings of the sub and revisiting most of the titles with mass amount of bass either music or movie I have noticed that the only thing missing with this sub is that it just doesn't go low enough (for my taste) guess I'm suffering from 15'' sub withdrawal, I'm curious in the audio differences with the ep500 and the ep800, can anyone help?

Never having a bigger sub, I'm loving my EP500 V4. It provides what seems like just enough bass for movies and music, that is with the volume knob at approx 30%. I'm not caring for any rattling the dishes effects. I've recently moved up from an EP350 V3. EP500 V4 is much better.

Sounds to me like you've got something wrong there. Hook an audio source directly up to the sub and see if it knocks you over (turn it down below 70% first!). Me, I suspect some kind of auto calibration jiggery pokery. You could also try turning off the auto-cal/room calibration stuff and see if it sounds different.